Mineral concentration



1965 D. H. DAHLEM ETAL 3,218,151

MINERAL CONCENTRATION 3 Sheets-Sheet 1 Filed March 4, 1963 Nov. 16, 1965 Filed March 4, 1963 D. H. DAHLEM ETAL MINERAL CONCENTRATION 3 Sheets-Sheet 2 I Nov. 16, 1965 DAHLEM ETAL 3,218,151

MINERAL CONCENTRATION Filed March 4, 1965 3 Sheets-Sheet 3 ORIGINAL POROUS DEPTH OF ZONE REDUCTION 3,218,151 MENERAL CONCENTRATION David H. Dahlem, Ann Arbor, Mich., and Will Mitchell,

Jr., and Charles L. Solleuberger, Milwaukee, Wis, as-

signors to Allis-Chalmers Manufacturing Company,

Milwaukee, Wis.

Filed Mar. 4, 1963, Ser. No. 262,773 12 Claims. (Cl. 75-1) This invention relates generally to a method of concentrating iron ore, and more particularly to a method of conditioning the fine grained nonmagnetic taconite ores causing grain growth, grain coalescence and partial conversion to magnetite so that the ores are readily amenable to magnetic concentration.

The taconite ores most profitably concentrated by this invention are the nonmagnetic iron oxide ores having microsized grains such as hematite and goethite. Large deposits of such ores are found in the Teal Lake area in upper Michigan and in the western half of the Mesabi Range in Minnesota.

Because of the fine grained nature of these ores they could not, heretofore, be concentrated economically. Since the iron oxide grains in these ores are of a size of about 3 to 5 microns, the necessary liberation grind would be too fine for commercial operation. Tests on these ores have further shown that even when a substantial liberation is achieved and the nonmagnetic oxide converted to magnetic magnetite, separation with magnetic separators is impossible because of the extreme fineness of this material.

As the high grade, more easily processed ores of the Mesabi Range are becoming depleted, more and more interest is being directed to the taconite ores of the Lake Superior area. To assure an adequate supply of iron ore, various types of commercial methods have been developed for concentrating taconite ores. However, the fine grained nonmagnetic oxide ores as hematite and goethite, to which this invention most directly relates, have not been susceptible to process by any of these commercial methods.

Various attempts have been made to alleviate this problem of fine grain size. Finer grinding to liberate the disseminated iron values is of course possible, but mechanical concentration is extremely dilficult if at all possible, in the minus micron size range. Other possibilities are to dissolve the iron minerals chemically or to use beneficiation processes, such as the Krupp-Kenn process, which smelt the ore. However, both of these methods are too expensive for commercial operation because of the low grade quality of the ore and large tonnages that must be handled. Thus, there is need today for a commercial procconcentrated profitably.

Accordingly, the primary object of this invention is to provide an inexpensive process for concentrating the finely disseminated nonmagnetic iron oxide minerals.

Another object of this invention is to provide a method whereby the finely disseminated nonmagnetic iron oxide ores such as hematite and goethite can be concentrated by a process readily adaptable to commercial operations.

Another object of this invention is to provide a process whereby the finely disseminated nonmagnetic iron oxide grains are caused to grow and coalesce in the solid state so that an increased liberation from the gangue may be obtained by crushing and grinding.

A further object of this invent-ion is to provide a process whereby iron oxide grains are caused to grow onto the particle surface as a magnetite cover.

Still another object of this invention is to provide a process whereby the finely disseminated nonmagnetic iron oxide ores can be enlarged and converted to magnetite by solid state reaction so that they are readily amenable to magnetic concentration.

nited States Patent 0 These and other objects shall hereafter become apparent from a full understanding of the following description, especially when considered in conjunction with the accompanying drawings in which:

FIG. 1 is a photomicrograph of a polished hematitic taconite sample before roasting;

FIG. 2 is a photomicrograph of the same sample after a four hour roast;

FIG. 3 is a photomicrograph showing a cross section of a polished ore surface before roasting;

FIG. 4 is a photomicrograph showing a cross section of the same surface area after one roast and quench cycle;

P16. 5 is a section drawing illustrating grain growth to the point of overflowing;

FIG. 6 is a sectional drawing illustrating a typical surface condition after a roast-quench-roast cycle; and

FIG. 7 is a phase diagram for the iron-oxygen-carbon and the iron-oxygen-hydrogen phases.

Essentially, this invention comprises a process whereby the finely disseminated iron oxide ores are processed by roasting so that they are made amenable to concentration by magnetic separation. In this process, the ores must first be crushed to at least a minus one-half inch particle size. Then the ore is roasted at a temperature range between 600 and 900 C. for a period of about one to four hours in an atmosphere of intermediate reducing strength as will be hereinafter described. This roasting will first cause an iron sponge to grow over the ore particle surfaces; second, cause the internal iron oxide grains to grow about ten fold; and third cause the iron oxide grains to be converted to magnetite. The ground ore is then quenched in a neutral or inert atmosphere to prevent reoxidation of the lower state iron oxide sponge coating, and re-roasted under similar roast conditions. The re-roasting converts the original iron oxide sponge to a solid coating of magnetite and may cause another sponge growth over'the magnetite coating. If the ore is then cooled and further ground to liberate the newly formed magnetite, the magnetite being much larger in grain size can easily be concentrated by magnetic separators. In most cases it would probably be necessary to deslime at about a minus 10 microns before magnetically separating the treated ore.

Prior art literature describes reduction roasting or magnetic roasting wherein nonmagnetic iron oxides are converted to magnetic magnetite. Also, grain growth of iron oxide minerals, per se, is old in the art. These prior art processes are based upon the phenomena of grain growth of magnetite in a plastic or fluid gangue at temperatures above 1000 C. In these teachings the iron atoms are shown to migrate freely through the fluid slag to effect the growth of the larger nuclei grains at the expense of the smaller grains.

What is new in this invention is the effected grain growth at temperatures substantially lower than Was believed possible from prior art teachings, and the remarkable magnetite layer that can be grown onto the ore particle surfaces at these temperatures.

Now we will proceed to examine these various growth phenomena in the order of their occurrence. The first noticeable change is the growth of the iron oxide sponge coating on the ore particles. The sponge growth appears to be independent of any other grain growth or reduction, and is formed rapidly and completely in a rather short time lapse (about one hour) before any other appreciable growth or reduction occurs.

The mechanism of the sponge growth appears to be quite different from the mechanisms of other grain growth phenomena. That is the spronge will grow over the ore surface covering gangue and iron oxide alike, and it will grow to a maximum thickness of about 20 microns when roasted for about one hour. This sponge growth,

unlike other growth phenomena, grows rapidly at the beginning of the roast to its maximum thickness of about 20 microns. No appreciable increase in thickness can be obtained with longer roasting retention times (up to four hours). While the atmosphere is critical for purposes of reduction, variations in atmosphere will not affect the thickness of the sponge growth. The one really critical parameter is the temperature range, as the sponge cover will not form at temperatures above 900 C. nor below 600 C. A satisfactory processing temperature is midway between the two limits, namely 750 C.

It is rather difiicult to study the exact structure and composition of the sponge cover since it is quite delicate and is easily wiped off the ore surface. The significance of the sponge cover is in its conversion to a solid magnetite cover upon re-roasting. This conversion of the sponge to magnetite will be discussed later.

The second growth phenomenon to occur is the actual grain growth of the iron oxide grains within the ore particles. Three to four hours of roasting at 750 C. is required to achieve an appreciable growth (ten fold). As explained above, grain growth of iron oxide grains is well known in the art at temperatures above 1000 C. since the iron and gangue atoms move about freely in the plastic or fluid gangue. However, we have found that grain growth of iron oxide grains occurs at temperatures as low as 500 C. by coalescence of smaller grains and general solid state recrystallization. Because of the immobility or rigidity of the gangue minerals at there lower temperatures (below 1000 C.) the grain growth proceeds by a somewhat different mechanism. At these lower temperatures the growth proceeds by concentrating the iron oxide at its original sites, and the rigidity of the gangue causes the growth to spill into voids or cavities and/or extrude over the ore surface. H6. 3 is a drawing which illustrates the nature of this growth in extruding over the ore surface.

FIGS. 1 and 2 are photomicrographs showing our grain growth. FIG. 1 shows the surface of a polished ore sample before roasting. The fine dissemination of the hematite (white grains) in a quartz gangue (dark gray area) are readily apparent. FIG. 2 is a photomicrograph of the same area after a four hour roast. (The sponge growth as described above had to be removed so that the original surface could be observed.) these two photomicrographs the grain growth is strikingly apparent. Further, comparison shows that the iron oxide grains also assume smooth or rounded edges thus losing their original feathered edges. The black areas in these two photomicrographs are voids and cavities, and comparison of FIGS. 1 and 2 illustrates conclusively that many of the voids and cavities shown in FIG. 1 were filled or partially filled with the iron oxide during the roast.

While this grain growth is taking place, or shortly thereafter, the iron oxide which is present as hematite or goethite (Fe O is reduced to magnetite (R This reduction would of course be necessary to enable concentration of the ore by magnetic separation.

During the process of reduction the roasting atmosphere is quite critical as only a partial reduction is sought. The reducing gas must be present in sufficient amounts to effect the reduction as well as grain growth.

There are three general types of reducing atmospheres in commercial use, namely, CO plus CO mixtures, H 9 plus H mixtures and combinations of the two. The relative ratios of CO zCO and H O:H may be varied to attain different reducing strengths. When using the CO zcO atmospheres, we have learned that atmospheres with less than five percent CO have such weak reducing potential that little reduction is effected and the grain growth is not appreciable. On the other hand, CO concentrations above ten percent have such strong reducing potentials that over-reduction to wustite (FeO) occur. Since wustite is nonmagnetic it is an undesirable product By contrasting if, and should be avoided. Thus to assure that magnetite is the only reduction product we found it necessary to maintain a reducing atmosphere having at least five percent CO with a CGZICO ratio of no less than 2.311 at 750 C. with the balance being nitrogen.

From the equilibrium diagram for the iron-oxygencarbon system in FIG. 7, it can be seen that the correct ratio of CO zCO to prevent over-reduction to wustite varies with temperature. At 600 C. the ratio can be no less than 1.2 and at 900 C. it can be less than 3.5. At 750 C. the lower limit is about 2.3. The upper limit for the ratio is fixed by the CO concentration used.

Similarly from the equilibrium diagram for the ironoxygen-hydrogen system, it can be seen that the correct ratio of H O:H to prevent over-reduction to Wustite varies with temperature. At 600 C. the ratio can be no less than 0.5 and at 900 C. it can be no lower than 4.5. At 0 C. the lower limit is about 1.8. The upper limit for the ratio is fixed by the H concentration used and a concentration greater than 5 percent would be desirable.

Mixtures of gases containing CO and H can also be used as would be obtained from a gas reformer or by underburning fuel. The combined concentration of CO and H should be greater than 5 percent and the ratio of CO and P1 0 to the CO and H respectively in the mixture should be no less than the foregoing values for the temperature being used. After the ore has been roasted for about four hours to effect the sponge cover growth, the grain growth and the reduction to magnetite as described above, the ore should be quenched in an inert or neutral atmosphere such as nitrogen to prevent reoxidation of the reduced iron oxides. For commercial operations, however, use of nitrogen may be rather expensive. Accordingly, the spent reducing atmosphere could be used for the quenching media when cooled.

Next, the ore, with its sponge coating, should be reroasted under the same roast conditions for another one to four hour period. In this re-roast a solid magnetite layer, having a thickness of 10 to 20 microns will replace most of the original sponge layer. This results from grain growth in the sponge, the sponge providing a porous network allowing ample space for growth. It is evident that this magnetite layer is produced within the sponge since this magnetite layer cannot be formed if the sponge cover has been removed from the ore before re-roasting.

FIG. 3 is a photomicrograph clearly showing a cross section of the magnetite layer {white cover) which has grown over a porous surface during a re-roast. FIG. 4 is a similar photomicrograph showing the magnetite layer grown over a polished gangue surface.

The outer magnetic layer usually overlays a porous zone. These two layers then have a total thickness of about 30 microns, or the thickness of the original sponge layer. Extended re-roasting retention times or multiple re-roasts will not increase the thickness of the magnetite layer beyond about 10 more microns, or the thickness of the original sponge layer.

The surface characteristics of the final product are illustrated in FIG. 6. The inner part of some of the larger grains may be unreduced hematite or goethite, but the smaller and average grains are usually completely reduced throughout. Moving out from this inner core we note an overlaying solid magnetite cover resulting from grain growth and reduction in the first roast. Over this reduced grain surface the spongelike coating covers the magnetic as well as the gangue (not shown). The final outer layer is a surface of magnetite resulting from the re-roast.

For study purposes the magnetic roasting was conducted in a 4.5 laboratory verticle tube furnace. For

commercial operation, however, it is possible that any of the commercial type roaster furnaces as a hearth furnace, shaft furnace, fluidized bed or rotary kiln may be used without deviating from the spirit of this invention.

Once the finely disseminated taconite ores are prepared by the magnetic roasting described herein, they can be processed by any of the present commercial magnetic separation processes.

The following procedure is one that could be followed. It should be understood, however, that it is only exemplary, as the specific grinding sizes necessary and other such data would vary with the specific ore being processed, the roasting furnaces being used and so on.

Preliminary to any roasting, the ore should first be ground to say at least a minus one-half inch. This would be necessary so that the roasting atmosphere could penetrate the entire mass of the ore. After such grinding the ore should be roasted at about 750 C. in an atmosphere containing from at least 5 percent CO with a CO :CO ratio of no less than 2.3:1, for a period of from one to four hours. As an alternative a reducing atmosphere of H O-H could be used if the H O:H ratio is not less than 1.8:1 at 750 C. A mixture of CO plus CO and H plus H could also be used provided the CO plus H are at least 4.5 percent of the total atmosphere and individual CO :CO and H O:II ratios are not less than the values given above for the single reducers.

Thereafter, the ore should be quenched in an inert atmosphere such as N or the cooled spent reducing atmosphere and then re-roasted. Additional quenching and re-roasting may or may not be necessary depending upon the grain growth and magnetite layer produced by the first roach-quench-roast cycle. After the roasting is complete, the ore should be further ground to about minus 300 mesh or any size Where substantial liberation of the larger grains can be obtained economically. Depending upon the type of ore it may be advisable to deslime the ore at about minus 10 microns. These slimes may interfere with magnetic separation and the magnetic grains in that size range would not be susceptible to magnetic separation. Thereafter, the ore would be amenable to separation by any of the available commercial magnetic separators or processes.

Having now particularly described and ascertained the nature of our said invention and the manner in which it is to be performed, we declare that what we claim is:

1. A method of reducing ground, fine-grained nonmagnetic iron oxide ore to magnetically separable magnetite comprising:

(a) roasting said ground ore at a temperature in the range of 600 to 900 C. in an atmosphere containing CO and from to percent CO with a CO :CO ratio in the range of at least 1.2:1 to 3.5:1 until an iron oxide sponge is formed over the surface of the ore, and said fine grains grow, and reduce to magnetite;

(b) quenching said ore in an insert atmosphere; and

(c) re-roasting said ore at a temperture in the range of 600 to 900 C. in an atmosphere containing CO and from 5 to 10 percent CO and a C0 :CO ratio in the range of at least 1.2:1 to 3.5 :1 until the outer portion of the sponge layer is converted to solid magnetite.

2. A method according to claim 1 wherein the reroasted ore is cooled and then further ground to achieve substantial liberation of grains.

3. A method according to claim 1 wherein the fine grained ore, before the initial roast, is ground to at least a minus one-half inch particle size.

4. A method according to claim 1 wherein the said ore is quenched in an inert atmosphere and roasted a third time at a temperature in the range of 600 to 900 C. in an atmosphere containing CO and from 5 to 10 percent CO with a CO :CO ratio in the range of at least 1.2:1 to 35:1 until a second solid magnetite layer is formed on the ore particles.

5. A method of reducing ground, fine-grained nonma netic iron oxide ore to magnetically separable magnetite comprising:

(a) roasting said ground ore at a temperature in the range of 600 to 900 C. in an atmosphere containing a mixture of H 0 vapor and H having an H O:H ratio of at least 1.8:1 until an iron oxide sponge is formed over the surface of the ore, and said fine grains grow and reduce to magnetite;

(b) quenching said ore is cooled and spent reducing atmospheres;

(c) re-roasting said ground ore in accordance with step (a) until the outer portions of the sponge layer is converted to solid magnetite.

6. A method according to claim 5 wherein the reroasted ore is cooled and then further ground to achieve substantial liberation of grains.

7. A method according to claim 6 wherein the fine grained ore, before the initial roast is ground to at least a minus one-half inch particle size.

8. A method according to claim 5 wherein the said ore is quenched in an inert atmosphere and roasted a third time in accordance with step (at) until a second solid magnetite layer is formed on the ore particles.

9. A method of reducing ground, fine-grained nonmagnetic iron oxide ore to magnetically separable magnetite comprising:

(a) roasting said ground ore at a temperature in the range of 600 to 900 C. in an atmosphere containing a mixture of CO CO, H and H 0 vapor with a CO zCO ratio of at least 2.311 With from 5 to 10 precent CO and a H O:H ratio of at least 1.8: 1, until an iron oxide sponge is formed over the surface of the ore and said fine grains grow and reduce to magnetite;

(b) quenching said ore in cooled and spent reducing atmospheres;

(c) re-roasting said ground ore in accordance with step (a) until the outer portion of the sponge layer is converted to solid magnetite.

10. A method according to claim 9 wherein the reroasted ore is cooled and then further ground to achieve substantial liberation of grains.

11. A method according to claim 9 wherein the fine grained ore, before the initial roast is ground to at least a minus one-half inch particle size.

12. A method according to claim 9 wherein the said ore is quenched in an inert atmosphere and roasted a third time in accordance with step (at) until a second solid magnetite layer is formed on the ore particles.

References Cited by the Examiner UNITED STATES PATENTS 2,711,368 6/1955 Lewis 1 2,747,988 5/1956 Von Haken 75l 2,945,755 7/1960 Schulz 75--1 3,005,699 10/1961 Erck et al 75--1 BENJAMIN HENKIN, Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,218,151 November 16, 1965 David H, Dahlem et a1,

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 5, line 51, for "insert" read inert line 54, for "and", second occurrence, read with column 6, line 15, for "is" read in line 37, for "precent" read percent Signed and sealed this 5th day of July 1966b (SEAL) Attest:

ERNEST W. SW'IDER Attesting Officer EDWARD J. BRENNER Commissioner of Patents 

1. A METHOD OF REDUCING GROUND, FINE-GRAINED NONMAGNETIC IRON OXIDE ORE TO MAGNETICALLY SEPARABLE MAGNETITE COMPRISING: (A) ROASTING SAID GROUND ORE AT A TEMPERATURE IN THE RANGE OF 600 TO 900*C. IN AN ATMOSPHERE CONTAINING CO2 AND FROM 5 TO 10 PERCENT CO WITH A CO2:CO RATIO IN THE RANGE OF AT LEAST 1.2:1 TO 3.5:1 UNTIL AN IRON OXIDE SPONGE IS FORMED OVER THE SURFACE OF THE ORE, AND SAID FINE GRAINS GROW, AND REDUCE TO MAGNETITE; (B) QUENCHING SAID ORE IN AN INSERT ATMOSPHERE; AND (C) RE-ROASTING SAID ORE AT A TEMPERATURE IN THE RANGE OF 600 TO 900*C. IN AN ATMOSPHERE CONTAINING CO2 AND FROM 5 TO 10 PERCENT CO AND A CO2:CO RATIO IN THE RANGE OF AT LEAST 1.2:1 TO 3.5:1 UNTIL THE OUTER PORTION OF THE SPONGE LAYER IS CONVERTED TO SOLID MAGNETITE.
 5. A METHOD OF REDUCING GROUND, FINE-GRAINED NONMAGNETIC IRON OXIDE ORE TO MAGNETICALLY SEPARABLE MAGNETITE COMPRISING: (A) ROASTING SAID GROUND ORE AT A TEMPERATURE IN THE RANGE OF 600 TO 900*C. IN AN ATMOSPHERE CONTAINING A MIXTURE OF H2O VAPOR AND H2 HAVING AN H2O:H2 RATIO OF AT LEAST 1.8:1 UNTIL AN IRON OXIDE SPONGE IS FORMED OVER THE SURFACE OF THE ORE, AND SAID FINE GRAINS GROW AND REDUCE TO MAGNETITE; (B) QUENCHING AID ORE IS COOLED AND SPEND REDUCING ATMOSPHERES; (C) RE-ROASTING SAID GROUND ORE IN ACCORDANCE WITH STEP (A) UNTIL THE OUTER PORTIONS OF THE SPONGE LAYER IS CONVERTED TO SAID SOLID MAGNETITE. 